Why does your Car Battery Voltage Drop Overnight?

Have you attempted to start your car in the morning only for your car to fail to start or crank slowly?

There are several reasons why your car may fail to start but a common one is a case of a dead battery, where the voltage dropped overnight and your battery does not have enough juice to start the engine.

While electrical accessories accidentally left switched ON can cause your battery to drain overnight, there are other possible causes.

In this post, I highlight:

  • the likely causes why your car battery voltage may drop overnight
  • how much voltage drop is normal for a good working battery
  • how you can quickly locate causes of battery drain , and
  • how you can stop your battery voltage from dropping overnight.

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Why your Car Battery Voltage Drops Overnight?

Your car battery voltage can drop overnight draining your battery for several reasons:

  • Leaving your car ignition ON: Is the car’s ignition accidentally left switched ON? This can drain your car battery in a few hours.
  • Accessories accidentally left on. Are there any accessories that may have been accidentally switched On? Interior dome lights, radio?

    Does your car have any accessories, usually aftermarket accessories that continue drawing power from the battery if not switched off even with the ignition off
  • Driving with many accessories switched On – headlamps, audio system, heater, fog lights slows down your car’s charging and your car battery voltage can be lower than usual
  • Bad connections at the battery terminals slow down the charging process of the batteries
  • Are you only taking your car for short trips? Short trips may not be enough to help your battery charge fully especially if the car has been parked for several days.
  • An aging battery. As batteries age, they lose their ability to hold the charge and can quickly drain overnight.

    Has your battery been tested – how many CCAs is it able to deliver compared to what it is designed to supply?

    If it is only able to supply a fraction of what it is designed to, then this is your confirmatory test that the battery is failing and should be replaced.
  • A bad or failing alternator. A working alternator should generate about 14.5V measured across the battery terminals with a digital meter with the car idle.

    If it is considerably higher or lower, then your alternator is probably faulty.

    Check out our guide on what basic tests you can complete confirming the functionality of your alternator.>>
  • Batteries self-discharge. batteries also lose some of their charges even without any appliances connected, through a self-discharge process.

    This though is not a significant drain on the battery capacity especially overnight.

    A 55Ah battery may lose less than 0.1% of its capacity overnight as an example.

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What is Causing your Car battery to Drain Overnight?

You can connect a digital multi-meter set to measure mA from the battery with the car’s ignition switched off.

Ideally, the reading you get should be about 30mA. If you measure a higher current draw, then quite likely you have an electrical accessory that is draining your battery’s power.

To locate the accessory, quickly, you are probably going to need someone to help you monitor the meter reading while you remove the car fuses one by one until the current draw is zero.

The offending accessory is connected to the last fuse you removed dropping the current to zero.

Check if it has an ON/OFF switch that needs to be switched off even with the ignition switched off

You may also be interested in: What can Cause the Battery to Drain when the Battery is OFF

How many Volts Should a Car Battery Lose Overnight?

There is no hard and fast rule on this one but a fully charged car battery losing more than 0.3V DC or over 0.036 Ah after being fully charged (after removing surface charge) indicates a problem with either the battery or in the car’s electrical system.

The 0.4 Ah assumes a maximum draw of 30mA at an average battery voltage of 12 V, 0.03mA x 12V = 0.36Ah

This measure is only a guide and should also factor in the surface charge on the battery.

Car Battery Surface Charge

For example, a fully charged battery immediately after driving might read as much as 12.8 VDC after driving which then drops to about 12.6 VDC after 10 or minutes – the fully charged battery voltage.

Should this voltage drop to below 12.3V DC overnight then there is likely a problem with either your battery or the car’s electrical system.

How to Stop your Car Battery from Draining Overnight?

To reduce the risk of your car’s battery draining overnight, you should:

  • Connect your car battery to a maintenance charger.

    If you have been driving with many electrical accessories switched on that have a high current draw, chances are that your car battery has not been charged as well as it should.
  • Switch off all electrical accessories – audio system and others that may continue drawing significant power from the battery even after the ignition is switched off.
  • Plan to replace your car battery before its specified lifespan.

    Arrange to have your car battery tested by a qualified auto technician to confirm if it is still able to deliver the expected power when you notice signs of a slow start of the engine or close to its design life.

    Replace your car battery immediately it shows signs of failure.


If your car’s battery voltage drops overnight, check that you do not have any electrical accessories accidentally left on – this is the most common problem.

If you have been driving with many accessories switched on, these can also slow down your battery’s charging and increase the risk of draining flat.

An aging battery also drains fast. To stop your battery voltage from draining overnight, test and if necessary replace batteries that are showing signs of failure. Also, always connect your car’s battery to a maintenance charger if there is a risk of the battery not charging well – long park times with only short drives to charge the battery.

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